You don want to change the filters. Simply rinse them in old tank water while doing your water changes. Wait until the filters are literally falling apart in your hands.
As for your parameters you shouldn't be seeing any nitrites or ammonia in a cycled tank. Have you stirred up anything recently? Gravel vac etc?
Have you tested tap water? What type of conditioner are you adding during water change?
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley